Larger-capacity and longer-distance optical fiber communication has progressed due to technologies such as high-speed intensity modulation signals and wavelength multiplexing. In addition to the technologies, in recent years, technologies such as polarized light multiplexing and multi-level phase modulation have been used due to the improvement of digital signal processing technology. In addition, communication capacities have been able to be still more increased utilizing existing optical fiber networks.
Such circumstances have caused demands for the higher integration, downsizing, higher functions, lower costs, and the like of optical communication devices used in optical fiber communication. There have been trade-off relationships between the demands, and it has been difficult to satisfy the demands in line with the conventional technologies.
The size, performance, and the like of an optical communication device depend greatly on the configuration and performance of an optical waveguide included in the optical communication device. Thus, production of optical waveguides has proceeded using manufacturing processes for semiconductor integrated circuits. Specifically, an optical waveguide is produced by designing fine optical waveguide patterns on a semiconductor thin film. Semiconductors, e.g., silicon and the like, have higher refractive indices than glass, and therefore, facilitate the downsizing of optical waveguides in comparison with glass waveguides. Further, the utilization and conversion of high-definition complementary metal oxide semiconductor (CMOS) process technology used in manufacturing of large scale integrations (LSIs) can be expected.
An example of such semiconductor (silicon) optical waveguides is described in each of PTLs 1 to 3, and the like. A silicon optical waveguide according to PTL 1 includes: a substrate including an insulator such as glass; a flat-shaped silicon thin film referred to as a base; and a rectangular waveguide including silicon and having a rectangular shape. The shape of the silicon optical waveguide is designed so that a predetermined relational expression between the width of the rectangular waveguide, a height upward from the substrate, and the thickness of the flat-shaped silicon thin film holds. As a result, the wavelength shift between TE and TM of guided light propagating through the silicon optical waveguide can be allowed to be less than 0.2 nm.